1. Field of the Invention
This invention relates to a process wherein a fluid stream comprising a noxious gas, i.e., hydrogen sulfide (H.sub.2 S), is treated with a water-soluble polymeric chelate of a polyvalent metal to oxidize the H.sub.2 S, i.e., the sulfur contained in the H.sub.2 S, to elemental sulfur. The elemental sulfur is separated and the polymeric chelate of the reduced metal in the aqueous solution is concentrated by means, such as dialysis or ultrafiltration. The polyvalent metal chelate is then oxidized and recycled. More specifically, the present invention relates to a process of removing H.sub.2 S from a gas stream using an aqueous solution of a water-soluble polymeric chelate of iron in the Fe.sup.+3 state, separating the elemental sulfur, concentrating the aqueous solution of the chelate of divalent iron using ultrafiltration or dialysis, oxidizing the Fe.sup.+2 to Fe.sup.+3, and recycling the polymeric chelate.
2. Relevant Art
It has been reported in U.S. Pat. No. 4,123,506 and in U.S. Pat. No. 4,202,864 that geothermal steam containing H.sub.2 S can be purified by contacting the steam with a metal compound that forms insoluble metallic sulfides. The subsequent disposal of the metallic sulfides is expensive and time consuming.
In U.S. Pat. Nos. 4,414,817 and 4,468,929, R. T. Jernigan discloses the treatment of geothermal steam by condensing it with an aqueous solution of a monomeric ferric chelate, such as N-hydroxyethylenediamine triacetic acid. The hydrogen sulfide is converted to elemental sulfur and the ferric chelate is reduced to ferrous chelate. The ferrous-monomeric chelate is subsequently oxidized to monomeric ferric chelate and re-cycled.
Z. Diaz in U.S. Pat. No. 4,400,368 discloses a cyclic process for the removal of hydrogen sulfide and carbon dioxide from a variety of gas streams. The "sour gas" stream containing these acidic gases is contacted with an aqueous solution of specific monomeric ligands or chelates of polyvalent metals or mixtures thereof. Hydrogen sulfide is oxidized to elemental sulfur, and the reactive metal monomeric chelate is reduced. The process includes sulfur removal and regeneration of the reactive metal chelate. Further, no provision is disclosed for removal of low molecular weight products or water from the process.
R. B. Thompson in U.S. Pat. Nos. 4,189,462 and 4,218,342 discloses a composition for use in an oxidation-reduction process for effecting the catalytic oxidation of hydrogen sulfide gas comprising a monomeric soluble ferric ion salt. This reference does not disclose a process using organic polymeric chelates of iron.
F. Engelhardt et al. in U.S. Pat. No. 4,518,745 disclose a number of metal chelates of water-soluble copolymers. The copolymers are useful as dyestuff auxiliaries and leather retanning agents.
M. E. Klecka in U.S. Pat. No. 4,518,577 discloses a process for the removal of H.sub.2 S from a sour gas stream. The H.sub.2 S is oxidized using aqueous solutions of oxidizing polyvalent metal chelates of specified monomeric organic acids and by the separation and recovery of the sulfur from solutions of specific organic extractants.
Z. Diaz in U.S. Pat. No. 4,518,576 discloses a cyclic process for the removal of hydrogen sulfide from a variety of gas streams. The sour gas stream is contacted with an aqueous solution of a monomeric chelate of iron (III) and a combination of a crystal modifier of phosphate and thiosulfate ions. The H.sub.2 S is oxidized to elemental sulfur and the thiosulfate reactant is reduced. The process includes sulfur removal and regeneration and recycle of the reactant.
C. A. Lieder et al. in U.S. Pat. No. 4,332,781 disclose a process for the removal of hydrogen sulfide and carbonyl sulfide from a gas stream in a staged procedure characterized by conversion of the hydrogen sulfide to produce sulfur in aqueous solution or suspension. The reactant materials include polyvalent metals bound to monomeric chelates.
J. O. Porath in U.S. Pat. No. 4,423,158 discloses introducing a chelate-forming group into a polymer such as polystyrene. There is obtained an adsorbant for bivalent or multivalent metal ions which are useful in ion exchange chromatography. The patent does not disclose the use of water-soluble polymeric chelates or the process of the present invention which are employed to remove H.sub.2 S from fluid streams with recovery of the polymeric chelate and active metal ion.
Additional art of interest includes, for example, U.S. Pat. No. 4,400,361.
Thus, several techniques are known for treating the exhaust stream from a geothermal steam well, or the sour gas or sour water from a refinery process with a monomeric organic chelate of iron (III). A problem with these processes is that the efficient separation of excess water, products and by-products which accumulate in the processes from the monomeric organic chelate of iron (II) or iron (III) from the aqueous stream is usually difficult and very costly. A portion of the expensive iron-monomeric organic chelate is lost during separation of water and the low molecular weight materials, and cannot be recovered. It is therefore highly desirable to have a process where the expensive organic iron (III) or iron (II) polymeric chelate can be separated easily from a waste stream originally containing H.sub.2 S, and recycled again and again. This invention provides such an improved process and a water-soluble polymeric composition for use in the process.